Self-balancing low temperature refrigeration system

ABSTRACT

Extremely low temperatures, in the range of -40*F. to -300*F., are achieved in a single circuit compression refrigeration system which is capable of being operated by a conventional compressor without the aid of special start-up or stand-by equipment. The system relies upon a series of intermediate cooling stages in which each stage includes the steps of withdrawing a portion of the liquid condensate from a compressed vapor-liquid refrigerant mixture which enters that stage, throttling the withdrawn condensate to a lower pressure, mixing the throttled condensate with refrigerant being recycled to the compressor from the final evaporator and evaporating the throttled condensate to absorb heat from and at least partially condense the compressed unconcensed vapor in the compressed mixture.

Missimer 1 Oct. 30, 1973 SELF-BALANCING LOW TEMPERATURE REFRIGERATIONSYSTEM [75] Inventor: Dale J. Missimer, San Anselmo,

Calif.

[73] Assignee: Gulf & Western Industries, Inc.,

New York, NY.

[22] Filed: Oct. 19, 1972 [21] Appl. No.: 299,049

[52] US. Cl 62/84, 62/114, 62/470, 62/473, 62/502 [51] Int. Cl. F25b1/00 [58] Field of Search 62/84, 114, 468, 62/470, 502

[56] References Cited UNITED STATES PATENTS 2,041,725 5/1936 Podbielniak62/114 UX 3,203,194 8/1965 Fuderer 62/l [4 3,487,653 l/l970 Myre 62/114X 3,668,882 6/1972 Sal'sten et al. 62/114 X l0/l972 Missimer 62/502 XPrimary Examiner-William F. O'Dea Assistant ExaminerPeter D. FergusonAtt0rney-Morton Amster et al.

[57] ABSTRACT Extremely low temperatures, in the range of 40F. to 300F.,are achieved in a single circuit compression refrigeration system whichis capable of being operated by a conventional compressor without theaid of special start-up or stand-by equipment. The system relies upon aseries of intermediate cooling stages in which each stage includes thesteps of withdrawing a portion of the liquid condensate from acompressed vapor-liquid refrigerant mixture which enters that stage,throttling the withdrawn condensate to a lower pressure, mixing thethrottled condensate with refrigerant being recycled to the compressorfrom the final evaporator and evaporating the throttled condensate toabsorb heat from and at least partially condense the compressedunconcensed vapor in the compressed mixture.

14,Claims, 3 Drawing Figures SELF-BALANCING LOW TEMPERATUREREFRIGERATION SYSTEM This invention relates to compression refrigerationsystems. More particularly, the present invention is concerned with anovel system for achieving a wide range of extremely low refrigerationtemperatures employing a mixture of refrigerants.

In the known systems of compression refrigeration, refrigerant vaporsare compressed, the vapors are condensed by heat exchange with ambientair or water, the condensate is throttled to a low pressure andevaporated to produce the refrigerating effect, and the refrigerantvapors are recycled to the compressor. When commercially availablesingle stage air conditioning or refrigeration compressors are employedin refrigera tion systems of the above type, such systems have beenlimited to achieving temperatures on the order of 40F. Where lowertemperatures have been required, the simple refrigeration system hasrequired substantial modification including the use of high pressure gassystems, expendable cryogenic refrigerants, specially designedmulti-stage compressors, or high pressure oilless compressors. Suchsystems are expensive to manufacture and operate and frequently requireskilled personnel in constant attendance.

Relatively low refrigeration temperatures have been achieved byemploying two or more complete refrigeration circuits in cascadeconnection. In such systems the evaporator of one stage forms a heatexchanger with the condenser of the next lower stage. Such systems havethe disadvantage of both size and expense in that each stage of thecascade includes all of the components of a complete refrigerationsystem. Moreover, such systems have not been effective in producingpractical systems due to freezing problems caused by circulation ofcompressor lubricating oils within the system unless the lower stageincluded the use of high pressure hazardous hydrocarbon refrigerants,special lubricants, a highly efficient oil separation system or othercomponents of a conventional cryogenic system.

A further method of achieving low refrigeration temperatures involvesthe use of a mixture of refrigerants in a single refrigeration circuit.publications and plublications describing such refrigeration techniques,in both opened and closed circuit systems, include U.S. Pat. No.3,203,194, issued Aug. 31, 1965 to A. Fuederer; U.S. Pat. No. 3,218,816,issued Nov., 1965 to Grenier; U.S. Pat. No. 2,041,725 issued May, 1936to Podbielniak; U.S. Pat. No. 3,487,653, issued Jan., 1970 to Myre; U.S.Pat. No. 2,952,139, issued Sept. 13, 1960 to Kennedy, et al. and AP.Kleemenko, One Flow Cascade System, Progress in Ref. Science &Technology, Permagon Press (1960). In such systems, low temperaturesareachieved by a series of intermediate cooling stages in which therefrigerant is partially condensed, the condensate is separated intovapor and liquid phases, the separated liquid phase is throttled, andthe throttled liquid is evaporated in heat exchange relationship withthe separated compressed vapors in order to achieve further cooling andfurther partial condensation. While such systems are capable ofachieving very low final refrigeration temperatures at relatively lowpressures and compression ratios, closed circuit systems employing thisconcept are not commercially practical unless they are equipped with arather complicated series of expansion tanks, control valves or otherdevices which will enable such systems to operate properly under allconditions. Specifically, it has been found that when these systemsoperate at higher temperatures, for example during start-up from roomtemperature or after power off and stand-by conditions, the refrigerantsemployed exert substantially higher vapor pressures and there is anincreased likelihood of reaching compressor discharge pressures andcompression ratios which would damage the conventional compressionequipment which such systems are designed to utilize. This difficulty isnot readily correctable by varying the amount of each refrigerantcharged to the system since the amounts of refrigerant which would bedesirable to avoid excessive start-up pressures are not the optimumamounts once the system begins to cool to its designed operatingtemperature level, i.e. the refrigerant charge is quite critical. Inaddition, because such systems employ throttling devices having aconstant or limited maximum mass flow for a given pressure andrefrigerant quality or vapor content as well as vaporliquid separatorsat each intermediate stage of condensation, the amount of condensateformed at each stage tends to be critical and may adversely affect theproper operation of the system either due to liquid holdup orinsufficient liquid flow. Indeed, such systems have exhibited a tendencyto undergo self-refrigeration, i.e. the lowest system temperature isreached at one of the intermediate cascade condensers rather than at thefinal evaporator, because liquid condensate formed up stream at apartial condensation step when throttled and evaporated reduces thedischarge pressure to a level so low that an inadequate amount of liquidcondensate is formed in the final step and thus, an insufficient amountof liquid condensate is available for feeding the throttling devicewhich in turn feeds the final evaporator. Although this situation. canbe corrected by the addition of more of the lowest boiling pointrefrigerant the addition of such additional refrigerant will upset thebalance required both for start-up and for final operating designtemperatures.

It is an object of the present invention to provide a novel improvedrefrigeration system for achieving a broad range of low temperatures.

It is another object of the present invention to provide a refrigerationsystem capable of achieving temperatures approaching the cryogenic rangein a single refrigeration circuit.

It is another object of the present invention to provide a novel,self-balancing refrigeration system employing a mixture of refrigerantswhich does not require the use of vapor-liquid separators and whicheliminates any criticality as to the amount of each refrigerantcharged'to the system or the amount of liquid condensate formed at anyintermediate point in the systern. I

A further object of the invention is to provide a novel refrigerationsystem capable of achieving extremely low temperatures at relatively lowdischarge pressures and compression ratios such that conventionalmassproduced air conditioning type compressors may be employed.

A further object of the invention is to provide a sealed self-balancingcompression refrigeration system employing multiple refrigerants andintermediate cooling stages containing a full refrigerant charge whichis capable of rapidly achieving low temperatures without the use ofcontrol systems, expansion tanks or other devices designed to alleviateproblems associated with high start-up pressures and compression ratios.

The above and other objects of the invention are accomplished in anovel, single cycle compression refrigeration system which employs amixture of nonflammable, non-explosive and relatively non-toxicrefrigerants having different boiling points and which includes at leastone, but preferably more than one, intermediate cooling stage in which acompressed mixture of the refrigerants is at least partially condensedto form a mixture consisting of compressed vapors and compressed liquidcondensate, a portion of the liquid condensate is withdrawn to feed athrottling device and the remainder of the liquid condensate ispermitted to flow downstream with the uncondensed vapor. Further coolingand at least partial further condensation is accomplished in eachintermediate cooling stage by evaporating the throttled liquidcondensate to absorb heat from and at least partially condensed theremaining vapor in the mixture of compressed refrigerant which includesboth vapor and the liquid condensate which was not fed through thethrottling device at that stage. This novel system may also includesub-cooling and oil separation apparatus as will be more fully describedherein.

It has now been discovered that by eliminating the successive stages ofvapor-liquid separation which have characterized prior compressionrefrigeration processes utilizing mixtures of refrigerants to achievelow temperatures, the criticality of the amount of each refrigerantcharged to a sealed system is eliminated and the system becomesinherently self-balancing, i.e. step wise partial condensation of therefrigerant mixture is achieved by a proper flow of liquid condensate toeach throttling device, the remainder passing downstream without theliquid hold-up which has characterized systems which employ separators.Moreover, the system of the invention is capable of achieving rapidcooling from start-up or stand-by conditions without excessive dischargepressures and compression ratios and without the use of bypass controldevices or expansion tanks to limit the amount of refrigerant in thesystem so as to avoid such pressures. While not wishing to be limited toany particular theory of operation, it is presently believed that theelimination of vapor-liquid separators allows the novel refrigerationsystem of the invention to be inherently self-balancing, i.e. to providefor the appropriate amount and flow of liquid condensate to eachthrottling device in the system and to vary the amount of that flow asdictated by changes in the operating conditions of the system, albeitthat a fixed amount of each refrigerant is initially charged into thesystem. This approach also appears to improve the thermodynamicefficiency of the system due to the fact that a more nearly equilibriumcondition between the vapor and liquid phases is achieved.

Irrespective of the theory of operation, it has been found that bywithdrawing to 95 percent of the liquid condensate formed in eachintermediate cooling stage rather than making a complete separation ofthe vapor and liquid phases the outstanding results heretofore describedare achieved. The exact amount of condensate withdrawn through eachthrottling device is not critical and will, of course, vary with thedesign and operating conditions of the system including the nature andamount of the refrigerants being employed, the type of throttlingdevice, the operating temperature and similar parameters, the control ofwhich are within the skill of the art.

The novel refrigeration system of the invention is not limited to usewith particular refrigerant combinations and a wide variety ofrefrigerant combinations may be employed to achieve operation over awide temperature range, e.g. 40F. to 300F. The refrigerants in anyparticular system will range from high boiling point refrigerants of thetype normally employed in conventional centrifugal type air conditioningsystems to extremely low boiling point refrigerants such as nitrogen,argon, neon, helium and the like. Preferably, the higher boilingrefrigerants will be selected from the group consisting of well-knownhalogenated hydrocarbons and their azeotropic mixtures. Hydrocarbonrefrigerants may also be used provided adequate safety precautions areemployed.

The boiling points of the individual refrigerants in the mixture must besufficiently far apart to permit step wise condensation at eachintermediate cooling stage of the system. Typically, each refrigerant inthe mixture will differ in boiling point from the next closest boilingrefrigerant by 50F. to 180F. The differences in boiling point betweenadjacent refrigerants may vary widely within these ranges but, ingeneral, may be smaller in those instances where a large number ofintermediate cooling stages are employed.

The relative amount and type of each refrigerant in the system is notcritical and ordinarily sufficient amounts of each refrigerant will bepresent to insure an adequate flow of liquid at each stage of theprocess when the system is in full operation. Among the consid erationsas to the type and amount of refrigerant charged to the system, as willbe apparent to those persons skilled in the art, are thedesign-operating temperature and pressures of the system; the nature ofthe condensing media; the size of various heat exchangers and throttlingdevices in the system; the compressor displacement and the nature of therefrigerants being employed. The optimum weight ratio of refrigerants inany particular system will also depend upon their respective molecularweights which influence their individual partial pressures; their liquiddensities; and the amount of liquid required at each intermediatecooling stage. In general, the amount of lowest boiling refrigerant willbe maintained at the minimum necessary to achieve the requiredrefrigeration effect of the system since higher amounts of the lowerboiling refrigerants tend to increase the discharge pressures of thesystem. In one typical system designed to operate at refrigeratingtemperatures as low as -230F., 22.5 mol percent trichlorofluoromethane(R-ll), 29.8 mol percent dichlorodifluoromethane (R-12), 15.0 molpercent chlorotrifluoromethane (R-13), 16.3 mol percentcarbontetrafluoride (R-l4) and 16.4 mol percent argon (R-740) wereemployed. The method by which the individual refrigerants are charged tothe system is not critical and charging can be based upon volume, weightor increase in partial pressure. Ordinarily, each refrigerant componentis added to the system separately in order of decreasing boiling point.

The system of the invention will be further understood by reference tothe accompanying drawings wherein:

FIG. 1 is a schematic representation of a refrigeration system havingthree intermediate cooling stages;

FIG. 2 is a schematic representation of a refrigeration system similarto that shown in FIG. 1 modified to include apparatus for additionalsub-cooling prior to the final evaporator; and

FIG. 3 is a schematic representation of a refrigeration system similarto that shown in FIG. 1 modified to include apparatus and a techniquefor removing compressor lubricating oils from the system.

Referring specifically to FIG. 1, a mixture of two or more refrigerantshaving different boiling points is charged into a single closedrefrigeration circuit generally identified as through a service valve 12or other conventional charging means such as a tube, pipe or the like,which will be sealed after the charging step. The amount of eachrefrigerant charged to the system may be predetermined by volume orweight or, in the case of lower boiling point refrigerants, by allowingeach refrigerant gas to circulate through the system until apredetermined partial pressure and a predetermined total pressure forthe system are reached.

Subsequent to the charging step, the vapors are aspirated by acompressor 14 and passed through conduit 16 to condenser 18 wherepartial condensation occurs.

Condensation occurs by heat exchange with ambient air forced overcondenser pipes 20 by a fan 22 or, alternatively, condensation may becarried out using a readily available source of water.

The partially condensed refrigerant mixture flows through conduit 24.toan auxiliary condenser 26 where, after the system is in operation,further condensation may occur by heat exchange with the cooler vaporsreturning to compressor 14 from the final evaporator 28 through conduit30. Utilization of an auxiliary condenser is not critical to therefrigeration system but such additional heat exchange at this pointserves to improve the thermodynamic efficiency of the system.

The partially condensed refrigerant mixture leaves auxiliary condenser26 through conduit 32 and passes to the first of a series of successiveintermediate cooling stages. The compressed mixture at this pointcomprises a liquid which is rich in the higher boiling refrigerant orrefrigerants and a vapor which is rich in the lower boiling refrigerantor refrigerants of the mixture. However, each fraction will contain atleast minor amounts of each of the refrigerants in the mixture. FIG. 1illustrates a refrigeration system including three intermediate coolingstages each consisting of a cascade condenser 34, 36 or 38, a throttlingdevice 40, 54 or 64 and associated conduits. The number of intermediatecooling stages in any system is not critical, provided at least one suchstage is present, and the selection of the ultimate number of stages,for example two to six stages, may be readily determined by thosepersons of ordinary skill inthe art depending upon the operating loadand other conditions for which the system is designed;

A portion of the compressed liquid condensate in conduit 32 is throttledin throttling device 40. Such throttling devices are well known in theart and may consist of a capillary tube, a thermal expansion valve, afloat valve or a similar device which permits the pres sure on theliquid flowing therethrough to be dropped from the discharge pressure ofthe system to the suction pressure of the system. Since the mass flow ofliquid through a throttling device is, inter alia, a function of theinlet pressure to the throttling device, it will be apparent to thoseskilled in the art that throttling. device 40 as well as the otherthrottling devices in the refrigeration system 10 will not be capable ofhandling the full flow of liquid condensate under all of the variety ofoperating conditions which may be encountered during operation of therefrigeration system. Accordingly, throttling device 40 is not designedto permit the flow of all of the liquid condensate in conduit 32therethrough. A portion of the compressed condensate, as well as thecompressed vapors formed in auxiliary condenser 26 flows through conduit42 to cascade condenser 34. The throttled low pressure liquid leavingthrottling device .40 passes through conduit 44 and is intermixed atpoint 46 with the cold vapors in conduit 30 which are returning. tocompressor 14 from final evaporator 28. Thereafter, this low pressuremixture flows through the portion of conduit 30 which is disposed incascade condenser 34 where the throttled liquid is at least partiallyevaporated and absorbs heat from the compressed mixture of liquidcondensate and vapor which entered cascade condenser 34 through conduit42 thereby at least partially further condensing the same.

It is a further and optional feature of the invention that a portion ofthe liquid condensate flowing in conduit 32 (or from successive similarconduits between intermediate cooling stages) may be split off fromconduit 32 through a suitable throttling device 48 and a conduit 50 andevaporated in evaporator 51 to obtain an independent refrigerationeffect. In such event that portion of the throttled liquid which isevaporated would by-pass cascade condenser 34 and be returned to conduit30 at a point located between cascade condenser 34 and auxiliarycondenser 26.

A tri-axial or three stream type heat exchanger may be used in place ofcascade condenser 34 and evaporator 51. In this case, valve 48 andconduit 50 would be eliminated and throttling device 40 would beselected so that the evaporating refrigerant flowing in the low pressureside of the new heat exchanger would be ata rate sufficiently great toabsorb heat from both the partially condensing high pressure stream andfrom the external load.

The at least further partially condensed compressed mixture obtainedfrom heat exchange in cascade condenser 34 passes to the. secondintermediate cooling stage through conduit 52. Thereafter, the cycledescribed in connection with the first intermediate cooling stage isrepeated, i.e. a portion of the compressed liquid condensate iswithdrawn and throttled through throttling device 54 and passes throughconduit 56 to point 58 of conduit 30 where the throttled liquid is mixedwith recycling vapors. A portion of the compressed liquid condensate aswell as the compressed uncondensed vapors flowing in conduit 52 arewithdrawn through conduit 60 and enter into cascade condenser 36 wherefurther at least partial condensation occurs by heat exchange with thethrottled liquid passing through the portion conduit 30 disposed withincascade condenser 36. a

The compressed further condensed vapor-liquid mixture from cascadecondenser 36 passes to the next successive intermediate cooling stage:through conduit 62 and a portion of the liquid condensate is throttledin throttling device 64, passed through conduit 66 to point 68 where itis mixed with cold vapors being recycled from the final evaporator 28through conduit 30. As before, the compressed uncondensed vapors and aportion of the compressed liquid condensate in conduit 62 is withdrawnthrough concuit 70 and passed to cascade condenser 38 where furthercondensation occurs as a result of the at least partial evaporation ofthe liquid throttled in throttling device 64. The further condensedcompressed mixture in cascade condenser 38 is withdrawn through conduit72 and passes through final throttling device 74 to the evaporator inlet76 which is at the coldest system temperature and essentially at thesuction pressure. The liquid condensate is partially or completelyevaporated in evaporator 28 to achieve the final refrigerationtemperature of the system. The refrigeration circuit is closed byreturning the vapors and any residual liquid from evaporator 28 throughconduit 30 back to compressor 14, the vapor being mixed with additionalthrottled liquid portions prior to its passage through each of thecascade condensers associated with each of the intermediate coolingstages as previously described.

FIG. 2 illustrates a modification of the refrigeration system describedin FIG. 1 wherein the condensate emanating from the final intermediatecooling stage through conduit 72 is subcooled prior to the finalevaporation stage. The operation of the compressor 14, condenser 22,auxiliary condenser 26 and the intermediate cooling stages is identicalto that previously de scribed in connection with FIG. 1. Sub-cooling isaccomplished by dividing the compressed condensate flowing in conduit 72into two streams and utilizing the first stream to sub cool the secondstream. More particularly, a conduit 78 is provided for drawing off asaid first stream and the first stream is thereafter passed tothrottling device 80 where the condensate is throttled to the suctionpressure of the system. The throttled liquid which flows into conduit 82is colder than the compressed condensate flowing into conduit 72.Conduit 82 discharges into said sub-cooler 84 and the throttled liquidis employed to further cool the compressed condensate flowing-in line72. The throttled and at least partially evaporated liquid leavessub-cooler 84 through conduit 86 and is mixed at point 88 with coldvapor from evaporator 28 being recycled to the compressor through line30.

The sub-cooled compressor condensate in line 72 (except for that portionwithdrawn through line 78) is passed to final throttling device 74 andthen to the final evaporator 28, all as previously described withrespect to FIG. 1.

FIG. 3 is illustrative of a refrigeration system similar to thatdescribed in FIGS. 1 and 2 in which the system has been modified toprovide for the use of a compressor lubricating oil admixed with therefrigerant and for the removal of that lubricating oil at a point inthe system which is well in advance of the lowest temperatures which thesystem is capable of producing. While the circulation of lubricating oilis acceptable in compression refrigeration systems operating atrelatively high final evaporator temperatures, it cannot be tolerated insystems operating at low temperature since good lubricants haverelatively high pour points and will not flow thereby coating heattransfer surfaces, clogging conduits throughout the circuit and possiblyresulting in inadequate compressor lubrication. In order to overcomethese problems a technique is provided for removing the lubricating oilfrom the refrigerant mixture which technique includes the steps ofadding a relatively high boiling separation fluid to the refrigerantmixture. The separation fluid has a boiling point which is about 35F. tol l5F., preferably 40 to F. higher than the highest boiling refrigerantin the refrigerant mixture and has a high degree of miscibility orsolubility with the lubricating oil being employed. In view of the highboiling point of the separation fluid, the first condensation step inthe refrigeration system depicted in FIG. 1 will result in condensationof substantially all of the separation fluid which, due to its highmiscibility and solubility will entrain substantially all of thelubricating oil. Thereafter, the compressed liquid condensate includingthe separation fluid and the lubricating oil may be separated from thecompressed vapors, throttled to essentially the suction pressure andrecycled to the compressor along with the vapors being recycled to thecompressor from the final evaporator.

A wide variety of relatively high boiling, oil miscible separationfluids may be employed in the oil separation method and the choice of aparticular fluid will depend on a number of factors including theboiling point of the fluid; the boiling point of the highest boilingrefrigerant and the nature of the lubricating oil. The preferredseparation fluids are halocarbons since these materials are relativelynon-toxic, non-flammable and non-explosive. Typical halocarbons arethose which are normally employed as a refrigerant in high temperaturerefrigeration systems and include such materials astrichlorotrifluoroethane, methylene chloride, trichlorofluoromethane,dichlorofluoromethane, dichlorotetrafluoroethane, or combinations ofthese materials, Ordinarily, the lubricating oil employed forlubrication of the compressor will be a hydrocarbon.

Ordinarily, enough separation fluid will be presentafter thecondensation step to insure an adequate fluid flow in the separationsystem andto insure that substantially all of the lubricating oil willenter into solution.

Referring specifically to FIG. 3, which illustrates the refrigerationsystem of the present invention including an oil separation step, amixture of refrigerants, separation fluid and compressor lubricating oilas above described is charged into the closed refrigeration circuitgenerally identified as through a service valve or other conventionalcharging means as previously described. Subsequent to the charging stepthe vapors are aspirated by compressor 102 and passed through conduit104 to condenser 108 where partial condensation occurs by heat exchangewith ambient air forced over c'ondenser pipes 110 by a fan 112 oralternate condenser means as described in connection with FIG. 1. As afurther option, the partially condensed refrigerant mixture may flowthrough conduit 114 to auxiliary condenser 116 where, when the system isin operation, further condensation may occur by heat exchange with thecooler vapors returning to compressor 102 from the final evaporator 1 18through conduit 120. The partially condensed refrigerant mixture leavesauxiliary condenser 116 through conduit 122 and passes to a vaporliquidseparator 124. The liquid at this point is rich in the separation fluidand compressor lubricating oil as well as the higher boilingrefrigerants while the vapor is rich in the lower boiling refrigerant orrefrigerants of the mixture.

The liquid separated in separator 124 passes through an optionaldryer-strainer 126 where particulate matter is filtered from the streamand residual moisture is removed and then through conduit 128 tothrottling device 130 which throttles the liquid, i.e. the pressure ofthe liquid drops from the discharge pressure to the suction pressure ofthe system. The throttled liquid next passes through conduit 132 topoint 134 of conduit 120 where the throttled liquid is mixed with vaporsbeing recycled from the final vaporator to the compressor. Thereafterthis mixture flows in conduit 120 through heat exchanger 136 where it isevaporated and used to absorb heat from and further partially condensethe separated vapors from separator 124 which enter heat exchange 136through conduit 138. Following the heat exchange the vapor in conduit120, which includesthe separation fluid and the lubricant, are recycledto the compressor through auxiliary condenser 116.

The compressed mixture of condensate and uncondensed vapor formed inheat exchanger .136 leaves exchanger 136 through conduit 140 and entersthe first of a series. of successive intermediate cooling stages. FIG. 3depicts two intermediate cooling stages 142 and 144. In each of theseintermediate cooling stages a portion of the compressed condensate isthrottled and used to cool and further condense a mixture consisting ofthe remainder of the compressed condensate and the compressed vapors allas previouslydescribed in connection with the system illustrated inFIG. 1. The compressed condensate leaving the final intermediate coolingstage 144 flows through conduit 146 to final throttling device 148, withor without sub-cooling as described in FIG. 2, and the throttledliquidfrom the final throttling device is partially or fully evaporatedin final evaporator 118 to achieve the final refrigeration temperatureof the system.

In lieu of the system described above and depicted in FIG. 3, thecondenser may be divided into two sections with a heat exchanger and thevapor-liquid separator installed between the sections. In thatarrangement, the vapors are aspirated by the compressor, passed througha conduit to the first condenser section where they are desuperheatedand partially condensed by heat exchange with ambient air (oralternately by water). The vapor and partially condensed mixture is thenpassed through the heat exchanger for further cooling and partialcondensation and then to the liquid-vapor separator. The condensedliquid at this point is very rich in the separation fluid and containsalmost all of the lubricating oils pumped by the compressor along withthe aspirated vapors.

The liquid separated in the separator passes through a conduit to athrottling device where the pressure is reduced to essentially thesuction pressure. This low pressure mixture of separation fluid andlubricating oil is then passed back through the heat exchanger and theseparation fluid is evaporated by counter-current heat exchange with thehigh pressure stream feeding the vapoi-liquid separator. The lubricatingoil and evaporated fluid is then recycled to the compressor suctionconnection.

The vapor mixture exiting from the separator is passed through a conduitto the second condenser section where additional heat is removed byfurther partial condensation and the condensed mixture is then fed tothe intermediate cooling stages as previously described.

The sizing of throttling devices, heat exchangers and other apparatusemployed in the system is not critical and will, of course, depend uponthe operating conditions for which a particular system is designed. Forexample, in determining the appropriate size of throttling devices suchas capillary tubes in which flow capacity is dependent upon the pressureand quality of entering condensate, the total weight of refrigeratant tobe circulated in the system is calculated and this amount is dividedbetween the various throttling devices in the system. Ordinarily, thethrottling device feeding the final evaporator will be designed tohandle about 30 to 50 percent of the total mass flow, the remainderbeing divided equally among the throttling devices feeding associatedwith each intermediate cooling stage. The optimum size of eachthrottling device is, of course, an empirical determination.

The optimum design for system heat exchangers is also an empiricaldetermination based on well known principles of heat and mass transfer.It has been found however that ordinarily the intermediate heatexchangers should be designed to handle about twice the evaporator loadfor systems having a final operating temper ature of about F. and fouror more times the load .parts of the system.

The invention will be further understood by reference to the followingillustrative examples:

EXAMPLE:

A mixture containing approximately 21.5 wt. percent (16.0 mol percent)trichlorofluo romethane (11-1 1), 21.5 wt. percent (18.2 mol percent)dichlorodifluoromethane (R-l2), (23.8 wt percent) (23.1 mol percent)chlorotrifluoromethane (IR-13), 30.2 wt. percent (35.0 mol percent)carbontetrafluoride (R-l4), and 3.0 wt. percent (7.7. mo1 percent) argon(R-740) was charged into the refrigeration system of FIG. 3 at roomtemperature. The system employed a conventional air conditioning typehennetic compressor and the final evaporator was connected to a cheststyle ultalow temperature freezer. After charging the system, therefrigeration system was completely sealed off so that the refrigerantmixture would freely circulate without loss. The initial charge pressurewas p.s.i.g.

The above-described system was started and permitted to run. Systempressure and temperature were periodically measured to determine theultimate operating characteristics of the system as well as the start-upconditions. The result of these measurements is shown in Table I.

As can be seen from the Table, the start-up pressures were 24 p.s.i.g.suction pressure and 365 p.s.i.g. discharge pressure (compression ratio9.75/1). Cooling commenced promptly and continued at a good rateuntil-178 F, the ultimate low temperature of the system, was reached injust over two hours. Subsequently, the system was shut down and thenre-started. No difficulty was encountered in again achieving theoperating characteristics set forth in Table I.

For the purpose of comparison, a refrigerant mixture identical to thatdescribed above, was charged into a refrigeration system which wasidentical to that shown in FIG. 3 with the exception that vapor-liquidseparators were employed between each cascade condenser. The balanced atrest initial charge pressure was 100 p.s.i.g. The results of the run areset forth in Table II.

TABLE [1 Air Suct. Disch. Time (from start) Temp. Press. Press. p.s.i.g.p.s.i.g.

0.0 hrs.(start-up) 72 F. 21 370 0.25 hrs. 52 F. 16.5 260 0.50 hrs. 34 F.12.5 205 0.75 hrs. 20 F. 180 1.00 hrs. 8 F. 8 155 1.25 hrs 2 F. 7 1401.50 hrs 11 F. 6.5 135 1.75 hrs. 19 F. 6 133 2.00 hrs. 28 F. 7.5 145"2.25 hrs. 47 F. 10 170 2.50'hrs 68 F. 2.75 hrs. 91 F. 3.00 hrs. 114 F.325 hrs 136 F. 3.50 hrs. 154 F. 3.75 hrs. 171" F. 12 170 4.00 hrs. -180F. 11 160 4.25 hrs. -183 F. 10 155 4.50 hrs. 185 F. 9.5 148 "4 wt. (5mol of carbontetrafluoride (compared to the total charge of the mixture)was added to the system at this point.

Table II reveals that the start-up pressure (370 p.s.i.g.) andcompression ratio (370/21.= 10.5/1) were higher for the system employingseparators. In addition, a comparison of Tables I and II indicates thatthe rate of cooling was much more rapid without the separators. Forexample, less than 0.75 hours was required to reach 25F. without the useof separators (Table I) while the same temperature was not achieved foralmost 2.00 hours using separators. Indeed, after 2.00 hours, the rateof cooling in the system employing separators was so low as indicated bythe extremely low suction pressure that it became obvious that a largeramount of a lower boiling refrigerant was required for proper operationof the system. Although the addition of carbon-tetrafluoride increasedthe suction and discharge pressures and the rate of cooling, that ratewas still only one-half as fast as the system of FIG. 3 and the ultimatelow temperature of 185F. was not achieved for 4.5 hours. At thistemperature, the system pressure was about 6.6 percent higher than thesystem of FIG. 3.

Of greater significance, as compared to the system of the invention, isthe fact that the system utilizing separators could not be restartedafter it had been shut down due to excessive start-up pressures.

In addition to the data listed in Table I & II above, a number oftemperature measurements were made throughout the systems, with bothsystems operating at the same discharge and suction pressures and withidentical refrigerant mixtures charged into them. Detailed analyses ofthese data show that approximately 60 percent of the liquid condensateformed by partial condensation in a cooling stage was withdrawn,throttled and mixed with the returning stream in the selfbalancingsystem of the invention. The result, in addition to the self-balancingeffect of the system at various operating conditions, was an improvementof 17.4 percent in heat transfer rate in each cascade condenser asdetermined by the reduction of the log mean temperature differenceacross each condenser.

A third system employing vapor-liquid separators between each cascadecondenser and additionally employing an auxiliary discharge vapor tanksimilar to that described at column 5, lines 31-19 of Fuderer U.S. Pat.No. 3,203,194 was employed. The discharge tank was connected to thesystem across the compressor so that excess high pressure vapors couldbe stored during start-up (with the low pressure connection closed andthe high pressure connection open). The system employed the samerefrigerant mixture at the same balanced at rest pressure as employedwith the system of FIG. 3. The results of this run are set forth inTable III.

*Vapors in auxilliary tank permitted to flow into system.

As compared to the system of FIG. 3, the results indicate that the rateof cooling in the system employing the auxilliary discharge tank wasabout one-half the cooling rate for the self-balancing system of theinvention until such time as operating conditions were reached whichmade it appropriate to permit the excess high pressure vapors to flowinto the system at the suction pressure (by closing the high pressureconnection and opening the low pressure connection between the tank andthe system). Once the full refrigerant charge was permitted to flow inthe system, operating results were comparable to the system of theinvention. However, use of the auxiliary tank was essential torestarting the system after shut-down.

These comparative examples illustrate that the system of the inventionis simplier to contruct and operate and achieves far superior operatingresults as comapred to mixed refrigerant systems which require completeb. partially condensing the compressed refrigerant vapor to form amixture consisting of compressed condensate and compressed uncondensedvapor;

c. subjecting the compressed mixture from step (b) to at least oneintermediate, cooling stage, each said intermediate cooling stageincluding the steps of throttling a portion of said compressedcondensate to a lower pressure, mixing said throttled condenate with themixture of refrigerants being recycled to the compressing step from thefinal evaporator, evaporating said throttled condensate to absorb heatfrom and at least partially condense the compressed vapor in theremaining mixture of the compressed condensate and compressed vapor,returning the mixture of evaporated, throttled condensate and recycledrefrigerant mixture to step (a), and passing said at least partiallycondensed compressed mixture to the next successive intermediate coolingstage;

d. throttling said compressed mixture obtained from the lastintermediate cooling stage to a lower pressure; i i

e. at least partially evaporating the throttled condensate produced instep (d) to produce the final refrigerating temperature and recyclingthe at least partially evaporated mixture of refrigerants to saidcompressing step.

2. The process of claim 1 whereinthe portion of compressed condensatethrottled in each intermediate cooling stage is determined by thecapacity of the throttling device.

3. The process of claim 1 wherein to 95 percent of the compressedcondensate entering each successive intermediate cooling stage isthrottled.

4. The process of claim 2 wherein said throttling device is a capillarytube.

5. The process of claim 2 wherein said throttling device is a thermalexpansion valve.

6. The process of claim 1 including 2 to 6 successive intermediatecooling stages.

7. The process of claim 1 wherein said refrigerant mixture comprises 2to 7 individual refrigerants.

8. The process of claim 7 wherein the difference in boiling otherrefrigerant in said mixture is 50F. to

9. the process of claim 7 wherein said refrigerant mixture includes atleast two halocarbon refrigerants,

10. The process of claim 7 wherein said refrigerant mixture includes atleast two halocarbon refrigerants and at least one inert gas selectedfrom the group consisting of argon, nitrogen and neon.

11. The process of claim 1 wherein only a portion of the cooling effectproduced by evaporating said throttled condensate in an intermediatecooling stage is employed to absorb heat from and at least partiallycondense the compressed uncondensed vapor and the remainder is employedto produce an external refrigeration effect.

12. The process of claim 1 wherein the compressed mixture obtained fromthe last intermediate cooling stage is divided into first and secondstreams, throttling said first stream to a low pressure, evaporatingsaid throttled first stream to absorb heat from and sub-cool said secondstream, mixing said throttled, evaporated first stream with refrigerantbeing recycled to the compressor from the final evaporator, throttlingsaid second stream to a lower pressure, and passing said throttledsecond stream to step (e).

13. The process of claim 1 wherein said refrigerant mixture furtherincludes a compressor lubricating oil and a separation fluid, saidseparation fluid having a boiling point which is 35 to F. higher thanthe highest boiling refrigerant in said. refrigerant mixture and beingsoluble in said lubricating oil, separating the compressed mixtureobtained in step (b) into liquid and vapor phases whereby saidlubricating oil and a substantial portion of the separation fluidremains in said liquid phase, throttling said separated oil-ladenseparation fluid to a lower pressure, mixing said throttled oil-ladenseparation fluid with refrigerant being recycled to the compressor fromthe final evaporator and passing said compressed vapor phase to thefirst intermediate cooling stage.

14. The process of claim 13 wherein the separated compressed vapor phaseis at least partially condensed by heat exchange with said mixture ofthrottled oilladen separation fluid and recycled refrigerant prior tosaid first intermediate cooling stage.

1. A compression refrigeration process employing a mixture ofrefrigerants having different boiling points, comprising the steps of:a. compressing a vaporous mixture of said refrigerants; b. partiallycondensing the compressed refrigerant vapor to form a mixture consistingof compressed condensate and compressed uncondensed vapor; c. subjectingthe compressed mixture from step (b) to at least one intermediate,cooling stage, each said intermediate cooling stage including the stepsof throttling a portion of said compressed condensate to a lowerpressure, mixing said throttled condensate with the mixture ofrefrigerants being recycled to the compressing step from the finalevaporator, evaporating said throttled condensate to absorb heat fromand at least partially condense the compressed vapor in the remainingmixture of the compressed condensate and compressed vapor, returning themixture of evaporated, throttled condensate and recycled refrigerantmixture to step (a), and passing said at least partially condensedcompressed mixture to the next successive intermediate cooling stage; d.throttling said compressed mixture obtained from the last intermediatecooling stage to a lower pressure; e. at least partially evaporating thethrottled condensate produced in step (d) to produce the finalrefrigerating temperature and recycling the at least partiallyevaporated mixture of refrigerants to said compressing step.
 2. Theprocess of claim 1 wherein the portion of compressed condensatethrottled in each intermediate cooling stage is determined by thecapacity of the throttling device.
 3. The process of claim 1 wherein 10to 95 percent of the compressed condensate entering each successiveintermediate cooling stage is throttled.
 4. The process of claim 2wherein said throttling device is a capillary tube.
 5. The process ofclaim 2 wherein said throttling device is a thermal expansion valve. 6.The process of claim 1 including 2 to 6 successive intermediate coolingstages.
 7. The process of claim 1 wherein said refrigerant mixturecomprises 2 to 7 individual refrigerants.
 8. The process of claim 7wherein the difference in boiling point between each refrigerant and theclosest boiling other refrigerant in said mixture is 50*F. to 180*F. 9.The process of claim 7 wherein said refrigerant mixture includes atleast two halocarbon refrigerants.
 10. The process of claim 7 whereinsaid refrigerant mixture includes at least two halocarbon refrigerantsand at least one inert gas selected from the group consisting of argon,nitrogen and neon.
 11. The process of claim 1 wherein only a portion ofthe cooling effect produced by evaporating said throttled condensate inan intermediate cooling stage is employed to absorb heat from and atleast partially condense the compressed uncondensed vapor and theremainder is employed to produce an external refrigeration effect. 12.The process of claim 1 wherein the compressed mixture obtained from thelast intermediate cooling stage is divided into first and secondstreams, throttling said first stream to a low pressure, evaporatingsaid throttled first stream to absorb heat from and sub-cool said secondstream, mixing said throttled, evaporated first stream with refrigerantbeing recycled to the compressor from the final evaporator, throttlingsaid second stream to a lower pressure, and passing said throttledsecond stream to step (e).
 13. The process of claim 1 wherein saidrefrigerant mixture further includes a compressor lubricating oil and aseparation fluid, said separation fluid having a boiling point which is35* to 115*F. higher than the highest boiling refrigerant in saidrefrigerant mixture and being soluble in said lubriCating oil,separating the compressed mixture obtained in step (b) into liquid andvapor phases whereby said lubricating oil and a substantial portion ofthe separation fluid remains in said liquid phase, throttling saidseparated oil-laden separation fluid to a lower pressure, mixing saidthrottled oil-laden separation fluid with refrigerant being recycled tothe compressor from the final evaporator and passing said compressedvapor phase to the first intermediate cooling stage.
 14. The process ofclaim 13 wherein the separated compressed vapor phase is at leastpartially condensed by heat exchange with said mixture of throttledoil-laden separation fluid and recycled refrigerant prior to said firstintermediate cooling stage.